Many metal oxides have historically been used in the manufacture of electrochemical capacitors including, but not limited to, RuO2, MnO2, V2O5, CoO and NiO. (Jang J et al., J Electrochem Soc 153:A321 (2006); Pang S et al., Electrochem Soc 147:444 (2000); and Liu et al., J Electrochem Soc 143:124 (1996)). Also, many of these metal oxide electrochemical capacitors undergo faradaic reactions as part of their capacitive behavior (i.e., psuedocapacitors). Such metal oxides also undergo redox reactions yielding mixed oxidation states, which is undesirable. In many instances, metal oxides are also more costly than nonmetallic oxides.
The field of ionic conductors and electron transport is discussed in Maier J, Nanoionics: ion transport and electrochemical storage in confined systems, Nature Materials 4:805-815 (Nov. 4, 2005).
However, there exists a long felt need for improved capacitors and ultracapacitors having more power density, particularly at high frequencies. There also exists a need for greater capacitance at higher charging/discharging frequencies. Improved energy storage (Joules/volume) is also needed.